Remove SourcePath.srcDir

This was always set to "." after aosp/1652613 and
aosp/1796650. While doing some rudimentary performance
profiling of soong, it turned out that SourcePath.String()
was taking 600+MB of memory joining the "." to the rest
of the path. Remove it for performance gains.

Bug: 262629589
Test: Verfied ninja files didn't change with this cl on aosp_arm64-userdebug
Change-Id: Id6b552ab8defe37e0c2b58e813fb1dbae427ae96